Conventionally, a derailleur for a bicycle comprises a base member, a movable member movable with respect to the base member and axially of the multi-stage sprocket assembly mounted to the bicycle, a feeding means for moving the movable member by use of an outside operation, and a guide mechanism supported to the movable member to guide the driving chain to one sprocket of the multi-stage sprocket assembly. Mainly, the outside operation through a control wire actuates the feeding means to move the movable member, so that the guide mechanism carries the driving chain axially of the multi-stage sprocket assembly and switches the chain from one sprocket to the other one, thereby performing a speed-change.
However, when the speed-changing is performed through the outside operation, especially for switching the chain from a smaller diameter sprocket to a larger diameter one, it often happens that any unskilled cyclist is apt to slowly perform the speed-change operation, and merely obtain insufficient operation magnitudes, so that the driving chain guided by the guide mechanism contacts or interferes on the way with the selected sprocket to make a noise and cause unexpected and unreasonable wear on the sprocket and the chain.